The present invention relates to an aluminum titanate ceramic and a process for producing said aluminum titanate ceramic. More particularly, the present invention relates to an aluminum titanate ceramic used in, for example, a head port liner, an exhaust manifold liner (these liners are for the heat insulation of the inside of an engine exhaust pipe) and a catalytic converter, as well as to a process for producing said aluminum titanate ceramic.
A ceramic using aluminum titanate as a base material has a low thermal expansion coefficient and a small Young's modulus. Hence, it is suitable for use in various members used under severe conditions in which the members are required to have high thermal shock resistance and low thermal expansion, for example, a head port liner, an exhaust manifold liner (these liners are for the heat insulation of the inside of a gasoline engine exhaust pipe) and a catalytic converter.
It is known that in ordinary aluminum titanate (aluminum titanate is hereinafter referred to as AT) ceramic materials, the Young's modulus and the strength are generally proportional to each other. That is, as the strength is higher, the Young's modulus is larger. Conversely, as the strength is lower, the Young's modulus is smaller. This is because when an AT ceramic material has a low strength, the crystal grains of the AT ceramic material are generally large and a number of cracks exist between the grain boundaries, allowing the AT ceramic material to be easily deflected and lowering its strength. Many of conventional AT ceramic materials have a Young's modulus of about 2,000 kgf/mm.sup.2 or more and a bending strength of 2-5 kgf/mm.sup.2.
Various improvements have been made for AT ceramic materials, depending upon their applications, by, for example, appropriately selecting the composition or adding appropriate additives. Japanese Patent Publication No. 5544/1985 , for example, proposes a silicate-containing aluminum titanate ceramic material made of raw materials comprising 50-60% by weight of Al.sub.2 O.sub.3, 40-45% by weight of TiO.sub.2, 2-5% by weight of kaolin and 0.1-1% by weight of magnesium silicate.
Japanese Patent Application Kokai (Laid-Open) No. 21756/1987 proposes an aluminum titanate-mullite ceramic material having the following composition:
MgO: 0.8 or less PA1 Al.sub.2 O.sub.3 : 53-74% PA1 TiO.sub.2 : 14-33% PA1 Fe.sub.2 O.sub.3 : 1.2-5% PA1 SiO.sub.2 : 6-20% PA1 CaO+Na.sub.2 O+K.sub.2 O: 0.3% or less
wherein the amount of vitreous phase in the mullite matrix of ceramic is 5% or less when measured on a section of ceramic.
Japanese Patent Application Kokai (Laid-Open) No. 164760/1989 proposes a shaped sintered material composed basically of aluminum titanate, which is produced from mixed raw materials consisting of 46% by weight or more of TiO.sub.2, 49.5% by weight or less of Al.sub.2 O.sub.3, 3-5% by weight of SiO.sub.2 and 0.2% by weight or less of impurities, the weight ratio of Al.sub.2 O.sub.3 and TiO.sub.2 being 1:0.95 to 1:1.05.
The ceramic proposed in Japanese Patent Publication No. 5544/1985 has a low Young's modulus and excellent casting property when used as a material to be cast by metal, but has the following problems. That is, the ceramic contains a magnesium silicate vitreous phase; when the ceramic is used in a heat cycle wherein high-temperature heating and cooling are repeated, the vitreous phase present at the grain boundaries moves during high temperatures; consequently, the grain boundary strength is reduced, cracks are developed at the grain boundaries, and there occurs deterioration in strength; thus, the ceramic has poor heat cycle durability. Further, the ceramic has low acid resistance owing to the presence of said vitreous phase.
The ceramic proposed in Japanese Patent Application Kokai (Laid-Open) No. 21756/1987 has excellent heat cycle durability and is useful as a material for use in a head port liner, etc. However, the ceramic, when containing mullite in a large amount, has a high Young's modulus and shows an inferior casting property, when used as a material to be cast by metal.
The ceramic proposed in Japanese Patent Application Kokai (Laid-Open) No. 16470/1989 has excellent heat cycle durability and acid resistance but, containing TiO.sub.2 in a large amount and consequently showing, when polished, a structure free from cracks, has a high Young's modulus and an inferior casting property.